Xerographic development apparatus



Feb. 18, 1969 w. A. SULLIVAN, JR 3,

XEROGRAPH IC DEVELOPMENT APPARATUS Filed D90. 27, 1966 INVENTOR.

WILLIAM A SULL IVAN,JR.

A TTORNEV United States Patent 3,428 025 XERO-GRAPHIC DEVELbPMENTAPPARATUS William A. Sullivan, Jr., Webster, N.Y., assignor to XeroxCorporation, Rochester, N.Y., a corporation of New York Filed Dec. 27,1966, Ser. No. 604,725 US. Cl. 118-637 Int. Cl. G03g 13/06, 15/08 5Claims ABSTRACT OF THE DISCLOSURE This invention relates in general toapparatus for developing electrostatic images, and in particular toapparatus for developing electrostatic images with a cloud of powdermaterial.

In the practice of xerography, a xerographic surface comprising a layerof photoconductive insulating material affixed to a conductive backingis used to create and support electrostatic images. In the usual methodof carrying out the process, the xerographic plate is electrostaticallycharged uniformly over its surface and then exposed to a light patternof the image being reproduced to thereby dissipate the charge in theareas where light strikes the layer. The undischarged areas of the layerthus form an electrostatic charge pattern in conformity with theconfiguration of the original light pattern.

The latent electrostatic image can then be developed with a finelydivided electrostatically attractable material such as a resinouspowder. The powder is held in image areas by the electrostatic chargeson the layer. Where the charge is greatest, the greatest amount ofmaterial is deposited; and where the charge is least, little or nomaterial is deposited. Thus, a powder image is produced in conformitywith the light image of the copy being reproduced. The powder maysubsequently be transferred to a sheet of paper or other surface andsuitably aflixed to thereby form a permanent print.

The electrostatically attractable developing material commonly used inxerography consists of a pigmented resinous powder referred to herein astoner. In order for the toner to attach itself to the electrostaticcharge corresponding to the image areas, the toner particles are chargedto a polarity opposite from the charge on the image areas.

One method for developing latent electrostatic images is the powdercloud system which is the system employed in the instant invention. Inpowder cloud development, a quantity or cloud of toner particles isbrought into an area adjacent a latent electrostatic image bearingsurface. In order for the toner to deposit itself on image areas, atriboelectric charge must first be imparted to the individual tonerparticles.

Charging of the toner can be accomplished by impacting the particlesagainst the walls of a tube or other container. By properly selectingthe material from the triboelectric series, triboelectric charging ofthe powder will occur during impact. The charged cloud of toner parti-3,428,025 Patented Feb. 18, 1969 ice cles may then be moved adjacent toa latent electrostatic image for development thereof.

In another method of charging toner the charge may be generated byagitating a quantity of carrier granules with the toner. The toner isusually a pigmented resinous powder. The carrier is usually a glas orsand bead coated with a material removed in the triboelectric seriesfrom the toner so that a triboelectric charge is generated between thepowder and the granular carrier upon mutual agitation. The tonerparticles thus become properly charged for development of images due totheir interaction with the carrier. The triboelectrically charged tonerparticles then can develop latent electrostatic images by contactingsurfaces containing the images.

In powder cloud development there is a natural tendency of random tonerparticles to become charged oppositely from the polarity intended. Whensuch occurs, the oppositely charged toner particles become attracted tonon-image areas and consequently manifest themselves as unwantedbackground.

The inability to create a cloud wherein all of the toner particles areof like polarity is thought to be due to mechanical shortcomings of theapparatus. In triboelectric toner charging there are invariably sometoner particles which receive incomplete or inadequate rubbing contactwith the charge generating carrier or tube walls. This incompletecontact and interaction results in incomplete charging of all tonerparticles which make up the cloud.

Regardless of the cause of improperly charged toner particles within thecloud, it is an observable fact that such errant toner particles doexist and their presence is a cause of undesired background in developedimages and final xerographic copy. The instant invention is directed toa development system capable of creating a powder cloud consisting ofunipolarly charged toner. Such unipolar charging can be achieved bysubjecting the toner to a sustained electrical discharge in the natureof a high density ion flow rather than by triboelectrification.

Therefore, it is an object of the present invention to develop latentelectrostatic images.

It is a further object of the present invention to improve powder clouddevelopment apparatus in xerographic machinery.

A further object of the present invention is to prohibit the generationof improperly charged toner particles in image developing powder clouds.

A further object of the invention is to form powder clouds wherein alltoner particles are charged to a like polarity.

A further object of the invention is to charge quantities of toner to animage developing polarity by means which are simultaneously capable ofimpelling the toner towards an electrostatic image to be developed.

Still a further object of the present invention is to create -a unipolarpowder cloud by introducing toner to an area of a sustained electricalbreakdown whereby the toner particles become charged by the high densityion flow within the breakdown.

These and other objects of the present invention are achieved by powdercloud development apparatus employing an apertured member positionedbetween an electrode and a latent electrostatic image bearing surface tobe developed. A high electrical bias between the apertured member andelectrode causes a sustained electrical breakdown therebetween. Whentoner is brought into this area it becomes charged due to the highdensity ion flow within the breakdown. As the toner is accepting thecharge, it exhibits a repulsive force with respect to the electrode.Because of this force, the breakdown may be simultaneously utilized tomove the charged toner away from the electrode towards the image bearingsurface for development thereof.

For a better understanding of the invention as well as other objects andfurther features thereof, reference is had to the following detaileddescription of the invention to be read in conjunction with theaccompanying drawings wherein:

FIG. 1 illustrates schematically a side cross-sectional view of axerographic machine employing the development instrumentalities of theinstant invention.

FIG. 2 is another side cross-sectional view with the developmentinstrumentalities enlarged for greater clarity.

Shown in the figures is an embodiment of the present inventionconstructed for continuous and automatic operation. The elements of thismachine, as shown schematically in the figures, are all conventional inthe xerographic art with the exception of the development station whichforms the basis of the present invention. For the purpose of the presentdisclosure, the several xerographic processing stations in the path ofmovement of the xerographic surface may be briefly described as follows:

A charging station A, at which a uniform electrostatic charge isdeposited on the photoconductive layer of the xerographic drum;

An exposure station B, at which the light or radiation pattern of copyto be reproduced is projected onto the drum surface to dissipate thedrum charge in the exposed areas thereof to thereby form a latentelectrostatic image of the copy to be reproduced;

A developing station C, at which toner particles are charged oppositelyto the charge of the electrostatic latent image and at which the chargedtoner particles are moved into contact with the drum surface, wherebythe toner particles adhere to the electrostatic latent image to form axerographic powdered image in the configuration of the copy beingreproduced;

A transfer station D, at which the xerographic powder iselectrostatically transferred from the drum surface to a transfermaterial or a support surface; and

A drum cleaning and discharge station E, at which the drum surface isbrushed to remove residual toner particles remaining thereon after imagetransfer.

The xerographic development apparatus is adapted to develop latentelectrostatic images which are formed in the conventional manner on adrum shaped xerographic surface 10. While the xerographic surface isshown as drum shaped, this is merely done for illustrative purposessince it is readily understood by anyone versed in the art that anysimilar xerographic surface such as a flexible belt or a rigid flatplate could readily be employed in the practice of the instantinvention.

The development apparatus of the disclosed embodiment is generallycontained with a housing 12. A bucket conveyor system disposed withinthe housing acts to lift a quantity of two component developer from thesump area 16 to an elevated location and then gravity drop the developeradjacent the development instrumentalities. Any suitable source ofpower, not shown, may be employed to move the conveyor along its path oftravel for lifting the developer.

Upon being dropped, the developer falls into a constricting set ofplates which begin to guide the developer through its path of travel.This set of plates may comprise an upper developer chute 18 in proximityto a guide plate 20. Positioned downstream from the upper guide plate 18is a lower guide plate 22 which further continues to guide the developerflow. As developer flows beyond the lower portion of the lower guideplate 22, it drops by gravity into the sump for further recirculationthrough the system. The chutes 18 and 22 and guide plate 20 extend for alength at least equal to the length of the drum. The sump and conveyorbuckets should also be of the same length to assure the flow ofdeveloper across a space equal to the length of the drum.

Spanning the space between the upper and lower guide chutes is agrounded conductive screen portion 24. The

mesh of the screen should be such that the individual toner particles ofa two component developer will be capable of passing therethrough. Themesh of the screen must, however, be fine enough so that the carrierparticles of the developer cannot pass therethrough. Positioned inproximity to the grounded screen 24 is a row of pin type electrodes 26.The electrodes are spaced from one another across the width of thescreen and drum so that When a potential is applied thereto a highdensity flow of ions is caused to span the space between the electrodesand screen across the entire length of the drum. So that the individualpin type electrodes may be provided with a source of potentialindependent of each other, the pins are mounted in an insulated block 28constructed, as for example, of any rigid dielectric or the like.

A source of potential 30 is electrically connected to the individualelectrodes for energization thereof. Since the electrodes must bemaintained at a high potential, a limiting resistor 32 may be insertedinto the circuitry between the electrode 26 and potential source 30 forsustaining the electrode at high potential.

In operation, when development is to be accomplished, latentelectrostatic image on a drum surface 10 is continuously moved adjacentthe development instrumentalities for sequential development of theimage. A quantity of two component developer material is then cascadeddown the chute 18, across the screen 24, down chute 22 and back to thesump for recirculation. When the potential is applied to the pinelectrodes 26 of sufiiciently high magnitude an electrical breakdown iscaused to occur between the electrodes and grounded screen. As thedeveloper is moved within this area, the high density ion flow caused bythe electrical breakdown achieves virtually complete charging of all thetoner particles in the path of the ion flow and repels them from thehigh potential source, through the screen and to the latentelectrostatic image to be developed.

It should be noted that the charging of the toner, according to theinstant invention, is not accomplished by a corona discharge. In acorona discharge a potential difference creates a limited flow of ionsand is evidenced by a bluish glow adjacent the corona emitting device.Corona emissions of this type are generally used to charge thexerographic surface and can in some instances cause a charging of tonerby the building up of a charge thereon. The charging of toner by coronadischarge has in the past produced less than desirable results. This isthough to be due to the properties of the toner in the path of a coronadischarge. If the toner is too conductive, it will dissipate the chargefrom the corona too quickly. If the toner is not sufiiciently conductiveit cannot take on the charge because of the low current output of acorona. Furthermore, corona charging of toner is limited in effect dueto the suppression of the corona dis charge by the toner after it hasbeen brought up to an equilibrium voltage with respect to the coronaemitting device. The equilibrium voltage for a system is generallysignificantly below the voltage of the emission source. The toner may bebrought up to such an equilibrium value but, thereafter, continuedcorona emissions are repelled from the toner by the toner charge whichhas already reached its peak potential for the system. In high densityion flow charging, as contemplated herein, the equilibrium potential isnever reached by the toner. Hence, the toner particles never ceaseaccepting charge from the emission source until they are moved outsideof the region of the ion flow.

According to the instant invention, however, a potential, higher thanthat used in corona charging, is applied in the charging area to cause atrue breakdown of the entire air space between the charge creatingelements. As such, the flow of ions is sufiiciently high to bombard thetoner and to create a virtually unipolar charge distribution on thetoner of significantly high magnitude to carry out high qualitydevelopment. An ion flow of this nature is also sutficient to charge thetoner to the point wherein it exhibits a repulsive force with respect tothe charging electrode for impelling itself towards the surface to bedeveloped.

Inasmuch as the electrical breakdown adjacent the pin electrodes andscreen creates a glowing spark in the vicinity, it is necessary to lightbaflle this glow from the photoconductive surface so that the light doesnot dissipate the image-representing charge on the surface 10. This isdone by providing parallel plates 34 and 36 as extensions of the chutemembers 18 and 22. By angling these baflles with respect to the straightline path between the electrodes 26 and drum surface 10, the lightcaused by the sparking electrodes is in effect shielded and does notreach the charge pattern on the xerographic surface 10. Since thebaffles will also act to deflect the flow of toner from its straightline path to the drum surface 10, a supplemental toner directingelectrode 38 may be employed. The electrode 38 extends the length of thedrum and is adapted to be biased by a source of potential 40 to the samepotential as that on the toner particles. As such, when the stream oftoner particles approaches the drum surface 10 it will be deflecteduntil it strikes the drum substantially perpendicular to the surface 10or radially with respect to the drum. This direction of toner movementagainst the drum has been found to offer the best development results. Asuitable insulating barrier 42 may be positioned between the electrode38 and baffle 34 for the sustaining of the potential on the theelectrode 38.

While the apparatus of the instant invention is operable within a widerange of variables, one set of parameters which may be employed to carryout the instant invention includes a positive 7500 volt DC. potential onthe potential source 30. A limiting resistor of approximately one megohmmay be employed to hold the pin electrodes somewhere between 0.50 andmilliamps. Since the charge breakdown is dependent upon the distancefrom which it is held from a grounded surface, a inch distance betweenthe pin electrode and grounded screen has been found satisfactory.

While pin electrodes have been shown as the desired breakdown creatingmeans, any similar electrode could be employed. It has been found,however, that a row of pin electrodes are more desirable than aconventional wire electrode since the pins are more susceptible forcreating and maintaining the high potentials for an extended durationneeded to practice the instant invention. Since each pin electrode underthese conditions is operative within a distance dependent radius, it hasbeen found that a M; inch spacing of the pin electrodes across thedevelopment zones works very well to keep a current flow across thelength of the drum for the inch electrode to screen spacing. Since thesystem is not dependent upon the triboelectric creation of charge on thetoner particles, a two component developer with an extremely high tonerconcentration may be employed. A developer with 3% to 4% toner has beenfound suitable. The carrier is added merely to facilitate handling ofthe toner.

It is obvious that this entire development system is readily adapted forincorporation in commercial xerographic machines.

While the present invention, as to its objects and advantages, has beendescribed herein as carried out in a specific embodiment thereof, it isnot desired to be limited thereby; but it is intended to cover theinvention broadly within the scope of the appended claims.

What is claimed is: 1. Apparatus for developing latent electrostaticimages formed on a xerographic surface with charged toner particlesincluding a conductive member positioned adjacent a xerographic surfaceto be developed, the conductive member being apertured to permit themovement of toner particles but not carrier granules therethrough,

toner charging means positioned in spaced proximity to the conductivemember, on the side thereof remote from the xerographic surface, meansto apply a potential to the charging means suflicient to cause asustained electrical breakdown from the charging means to the conductivemember, and

means to introduce a flow of two-component developer including carriergranules and charged toner particles into the space between the chargingmeans and conductive member whereby the toner will become additionallycharged by the high density ion flow created by the electrical breakdownand due to such charging, be electrostatically repelled from thecharging means towards the xerographic surface for development thereof.

2. The apparatus as set forth in claim 1 wherein the conductive memberis a screen and further including means to ground the conductive member.

3. The apparatus as set forth in claim 1 wherein the charging means is aseries of pin shaped electrodes and further including means to securethe pin shaped electrodes in a fixed position with respect to theconductive member.

4. The apparatus as set forth in claim 1 and further including bafllemeans positioned adjacent the side of the conductive member remote fromthe charging means, the baffle means being positioned to light shieldthe latent electrostatic image bearing surface being developed from theelectrical breakdown.

5. The apparatus as set forth in claim 4 and further including asupplemental electrode positioned adjacent the side of the baffle meansremote from the conductive member and means to electrically bias thesupplemental electrode to thereby deflect the flow of charged tonerparticles into a path substantially normal to the image bearing surfacebeing developed.

References Cited UNITED STATES PATENTS 3,306,193 2/1967 Rarey et al.118-637 XR 3,372,675 3/1968 Tressel 118-637 2,918,900 12/1959 Carlson118-637 3,263,649 8/1966 Heyl et al 118-637 3,295,440 1/1967 Rarey etal. 118-637 XR PETER FELDMAN, Primary Examiner.

US Cl. X.R. 117-175

